Sputtering method for forming superconductive films using water vapor addition

ABSTRACT

A superconducting film is deposited in a closed, non-baked reactor (10) containing a sputtering gas comprising O 2  with H 2  O vapor addition to preferably provide a total H 2  O vapor pressure of between 3×10 -3  mbar to 266×10 -3  mbar, where a copper oxide-based target material (15) is used and the deposition substrates (16) are maintained between 550° C. and 800° C.

The invention described herein was made in the performance of work undera U.S.A.F. contract identified as Air Force Contract F49620-88-C-0039,in which the Government has rights.

BACKGROUND OF THE INVENTION

Deposition of thin films by a variety of sputtering methods are wellknown, and taught by, for example, U.S. Pat. No. 4,361,472 (Morrison,Jr.) and U.S. Pat. No. 4,963,524 (Yamazaki). Gases present in theapparatus may include O₂, N₂, H₂ O, and CO₂, among others. So-called"in-situ," thin film sputtering techniques have been used to depositsuperconducting materials such as NbN and Nb₃ Ge utilizing heatedsubstrates, as taught by U.S. Pat. No. 3,912,612 (Gavaler et al.) andU.S. Pat. No. 4,043,888 (Gavaler), respectively. Electricallyconducting, highly transparent films of In₂ O₃ have also been sputtered,as taught by U.S. Pat. No. 4,400,254 (Freller et al.). There, watervapor with a pressure of between 3×10⁻³ mbar and 5×10⁻³ mbar was used asthe sole reaction gas, in combination with an inert gas and use oftemperature sensitive, water-cooled substrates. These films are used assemiconductors. It was speculated that the water vapor either furnishesO₂ for the oxidation of atomized In particles during decomposition inthe high frequency plasma, or liberates a reducing compound whichprevents excessive oxidation of the target and of the film beingproduced.

Since the discovery of YBa₂ Cu₃ O₇ (1:2:3 superconducting oxide), manytechniques have been investigated for preparing thin films of thecompound, for example, rf (radio frequency) sputtering on a roomtemperature Y₂ BaCuO₅ substrate material, as taught by U.S. Pat. No.4,929,595 (Wu). In Gavaler et al. in Physica B, Vols. 165 & 166, pp.1513-1514, Aug. 16-22, 1990, "Optimization of T_(c) and J_(c) inSputtered YBCO Films," both parallel and perpendicular rf and dcmagnetron sputtering of YBa₂ Cu₃ O₇ films are taught.

In Gavaler et al., Physica B, experiments were conducted in a non-bakedvacuum chamber, evacuated to a base pressure in the 10⁻⁶ torr range,using SrTiO₃, LaA10₃, MgO, and ZrO₂ substrates. Initially, pure O₂ or amixture of O₂ +Ar were used as the reactive sputtering gases. The mainresidual gas as a result of the use of a non-baked deposition chamberwas H₂ O. Values of T_(c) (critical temperature for superconductivity)of up to 91K were obtained. Water was deliberately added to thesputtering gas in certain parallel configuration sputtering tests in theamount of approximately 13×10⁻³ mbar (10 mtorr), With substratetemperatures at 670° C. In the parallel substrate-target configuration,film T_(c) 's of greater than 90K were obtained, which were generallyabout 5K better than films made without the addition of water under thespecific experimental conditions described. The same H₂ O addition usingthe perpendicular substrate-target geometry, which under the specificconditions described produced T_(c) values of greater than 90K withoutaddition of water vapor, produced no further improvement in the T_(c) 'sof the sputtered films, and in fact had, at that time the perceivednegative effect of producing Cu-O particles in the films.

There has been a long-felt need to provide YBa₂ Cu₃ O₇ films with goodand reproducible superconducting properties. There has also been a needto slow the deterioration of T_(c) 's of the films which are made fromtargets which have had long usage. It is one of the main objects of thisinvention to provide a process to solve such problems.

SUMMARY OF THE INVENTION

Accordingly, the invention resides in a method of preparing metallicoxide films on a substrate through the sputtering of copper oxide-basedtarget material onto a heated substrate in a closed reactor containing asputtering gas comprising O₂ and residual impurity amounts of H₂ O vaporhaving a pressure below approximately 1.0×10⁻³ mbar (0.77 mtorr),characterized in that the sputtering gas contains added H₂ O vapor, toprovide a total H₂ O vapor pressure in the reactor over that present asresidual impurity and up to 266×10⁻³ mbar, and a thin copper oxide-basedfilm having superconducting capability is in-situ deposited at asubstrate temperature of from 550° C. to 800° C., wherein substantialtarget erosion does not appreciably affect critical temperature valuesof the deposited film as deposition continues with time.

Preferably, the substrate will be disposed at a 90° angle to the targetsurface, the target will comprise a barium-copper oxide material, mostpreferably YBa₂ Cu₃ O₇, in pressed powder form, and the deposited filmwill be cooled slowly in oxygen. A useful addition of sputtering gasprovides a total pressure of from 100×10⁻³ mbar to 300×10⁻³ mbar. Auseful addition of water vapor provides a total H₂ O pressure of from17×10⁻³ mbar to 266×10⁻³ mbar. An effective sputtering gas within thereactor consists of (65 to 130)×10⁻³ mbar of O₂ +(25 to 75×10⁻³) mbar ofan inert gas such as Ar+(7 to 26)×10⁻³ mbar of H₂ O vapor.

The addition of the substantial amounts of water vapor described, overthat already present as a residual impurity in the reaction vessel,allows higher T_(c) values for deposited yttrium-barium-copper oxidesuperconducting films; allows consistently high T_(c) values even whenfilms are deposited from well used, eroded targets; allows deposition ata substantially lower temperature than the 670° C. standard minimumtemperature, allows the use of new substrate materials; allowselimination of standard silver paste as a thermal bond for thesubstrates; and finally, allows deposition of uniform quality film overa much larger substrate area than was previously possible.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention can be more clearly understood convenientembodiments thereof will now be described, by way of example, withreference to the accompanying drawing which shows a simplified partialschematic view of one embodiment of a dc magnetron sputtering apparatuswhich can be used in the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, a magnetron sputtering apparatus 10 isshown, with a pump 11 and associated valve 12 to provide a vacuum in thereactor chamber 13, and inlet valve 14 to allow flow of a sputtering gasinto chamber 13. The apparatus is not itself heated, being of anon-bakable design, only the substrate and associated support areheated. The target 15 is of the same composition as the film desired tobe sputter coated onto one or more coating substrates 16. The targetcomposition can be selected from a copper oxide-based material,preferably a barium-copper oxide based material, and most preferablyYBa₂ Cu₃ O₇, a material well known in the art, usually in pressed powderform. The target 15 is usually held in place by some means, such asclamping 17. The substrates 16 are usually held in place by a highlyheat conducting metallic paste 18, in the past usually a silver paste. Aresistance or other type heater 19, is used to heat the substrates 16,through the metallic paste 17 and metal support 20.

In the arrangement shown, the substrates 16 are perpendicular to theface of the target 15. Other geometries can be used, for example, thetarget and heater could be placed at point 21, parallel to the face ofthe target 15. The distance B defines the distance from the centerlineA--A of the target 15. Magnets 22 and 23 operate to establish electricand magnetic fields 24 which enhance ionization of the sputtering gasand focusing of the resulting plasma within the reactor chamber,allowing bombardment of the target by the ions, as is well known inconnection with magnetron sputtering devices. This bombardment willerode the target material, causing a portion of the target material todeposit on the heated substrates 16. An example of areas of erosion areshown at 25.

In operation, the target 15, such as a pressed YB₂ Cu₃ O₇ powder, andceramic substrates 16, such as LaAlO₃ (100), SrTiO₃ (100), MgO(100),ZrO₂ (100), Al₂ O₃, and the like, preferably in single crystal form, arepositioned within the closed reactor 10, which is evacuated to a (mostlyH₂ O vapor) pressure of 1.3×10⁻⁶ mbar (1×10⁻⁶ torr). A preferredsubstrate is LaAlO₃. Residual H₂ O vapor would increase in pressure upto 1.0×10⁻³ mbar, when the substrates are heated to about 670° C. priorto deposition. Then, an O₂ containing gas would be fed into the closedreactor 10, preferably either pure O₂ or a mixture of O₂ with Ar or anyother inert gas with the pressure in the reactor after the substratesare heated of preferably from 3×10⁻³ mbar (2.3 mtorr) to 266×10⁻³ mbar(200 mtorr), most preferably from 7×10⁻³ mbar (5 mtorr) to 30×10⁻³ mbar(40 mtorr).

The substrates can be heated to from 560° C. to 800° C. and thesputtering power can be between 25 watts and 150 watts. In thisinvention, excellent deposition can occur at from 560° C. to 650° C.,allowing use of less expensive substrates and the deposition of filmswith a different crystal orientation. For example, films with an a-axisorientation, which grow preferably at less than 600° C. (on LaAlO₃substrates), are preferable for Josephson Junction devices. Films with ac-axis orientation grow preferably (on LaAlO₃) at higher temperatures,that is over 670° C., and are preferred for microwave devices.

After the substrates are heated and the sputtering gas introduced, thefilms are deposited. Sputtering is continued until the desired filmthickness occurs on the substrates 16. The sputter deposited film willhave substantially the same composition as the target material, and inthe case of YBa₂ Cu₃ O₇ will be superconducting after cooling in O₂ ;that is, the film need not be post annealed at high temperatures (about850° C).

After deposition, the substrate temperature will be lowered slowly,preferably from 50° C./hr to 200° C./hr, so that the stoichiometry ofthe film will not become deficient in oxygen or, if deficient, will havesufficient time to pick up oxygen. The films produced will range inthickness from 20 nm (0.02 micrometer) to any desired thickness.Preferably, the distance of the substrate 16 from the centerline A--A ofthe target will be from 4 cm to 8 cm in the perpendicular configuration,and from 4 cm to 8 cm from the target surface in the parallelconfiguration. The method of this invention is applicable to either rfor dc magnetron sputtering.

The invention will now be illustrated with reference to the followingExample, which is not to be considered limiting in any way.

EXAMPLE

An off-axis, non-heated, magnetron sputtering reactor, somewhat similarto that shown in the simplified drawing, was used to deposit YBa₂ Cu₃ O₇films. The reactor was evacuated to a low vacuum of approximately1.3×10⁻⁶ mbar (1×10⁻⁶ torr), leaving residual, impurity H₂ O vaporhaving a pressure of approximately 1.3×10⁻⁶ mbar when the reactor iscold but which can rise to a pressure of 1.0×10⁻³ mbar when the reactorbecomes heated during the heating of the substrate to 670° C. Thesubstrate was a 1 cm×1 cm×0.05 cm thick, single crystal LaAlO₃ materialheld at a temperature of 670° C. The substrate was held in place bysilver paste on a nickel block.

Then a sputtering gas consisting of 130×10⁻³ mbar (100 mtorr) O₂+65×10⁻³ mbar (50 mtorr) AR+26×10⁻³ mbar (20 mtorr) H₂ O vapor was fedinto the reactor chamber. The total water vapor pressure in the chamberwas approximately 26.5×10⁻³ mbar. The target was 5 cm in diameter, 6 mmthick, pressed YBa₂ Cu₃ O₇ powder, consisting predominantly of thesuperconducting orthorhombic phase. The sputtering power was 60 watts,and the distance between the substrate surface and the centerline of thetarget was 5.8 cm. Sputtering continued until YBa₂ Cu₃ O₇ films having athickness of 100 nm (0.1 micrometer) were in-situ deposited, over a6-hour period, on the heated substrate.

The reactor and sputtered film were then cooled at a rate of 150° C./hr.The film had the 1:2:3 stoichiometry of YBa₂ Cu₃ O₇, with a T_(c) of 91Kand a J_(c) of 2×10⁻⁶ amps/cm² at 77K. Only minor amounts of Cu-Oparticles were observed. These results were reproducible over a longtime period even using substantially eroded targets. By deliberatelyadding water vapor to the sputtering gas, deposited films with T_(c) 'sof over 90K have been reproducibly deposited from a highly eroded targetwhich, without the addition of water vapor, was producing films withT_(c) 's of 80K and less.

A second important discovery was that the addition of extra water vaporallowed the deposition of high-T_(c) (90K) film over a much largersurface area than could previously be done. As an example, in oneexperiment substrates were mounted in the center--the 12 o'clock, the 3o'clock, the 6 o'clock, and the 9 o'clock positions--of a 5 cm diametersubstrate holder. Four YBa₂ Cu₃ O₇ films were then sputtered, usingadded water vapor as described previously. The T_(c) in every film wasgreater than 90K. This type of coverage could not be obtained previouslyeven in experiments in which the substrate holder was rotated and a newtarget was employed.

A third discovery was that high T_(c) films can be deposited at asignificantly lower substrate temperature when water vapor is added, aspreviously described, to the sputtering gas. Previously, it was foundthat the minimum temperature for producing the highest quality YBa₂ Cu₃O₇ films was about 670° C. In the past, reduction of only 20° C. to 30°C. caused T_(c) 's to drop to about 80K. Further reduction in substratetemperature produced films that were not superconducting.

We have now shown that with the deliberate addition of water vapor,within the range set forth, as previously described, films with T_(c) 'sas high as 88K can be obtained at a substrate temperature of about 600°C. The result could open the door to the use of new substrate materialswhich are stable at less than or equal to 600° C. Also, the ability ofobtaining high quality YBa₂ Cu₃ O₇ films over a much greater temperaturerange than previously possible eliminates the need to bond substrates tothe holder with silver paste. This method of mounting substrates hasbeen used in the past to ensure good thermal contact between substratesand holder and to maintain the substrate temperature within therelatively narrow range that up to now has been required to deposit thehighest quality films superconducting films. The method of thisinvention will also be applicable to other oxide-based targets anddeposited films.

We claim:
 1. In a method of preparing metallic oxide films on asubstrate through the sputtering of copper oxide-based target materialonto a heated substrate in a closed reactor containing a sputtering gascomprising O₂, and residual impurity amounts of H₂ O vapor having apressure below approximately 1.0×10⁻³ mbar, wherein the improvementcomprises a sputtering gas containing added H₂ O vapor, to provide atotal H₂ O vapor pressure in the reactor of from 7×10⁻³ mbar to 266×10⁻³mbar, and a thin copper-oxide based film having superconductingcapability is in-situ deposited on a substrate disposed at a 90° angleto the target surface, at a substrate temperature of from 550° C. to800° C., and slowly cooling the sputtered film at a rate of from 50°C./hr to 200° C./hr, wherein substantial target erosion does notappreciably affect critical temperature values of the deposited film asdeposition continues with time, and after cooling, the stoichiometry ofthe film will not be deficient in oxygen.
 2. The method according toclaim 1, where the target and deposited film consist of YBa₂ Cu₃ O₇material, the target material being in pressed powder form.
 3. Themethod according to claim 1, where the substrate temperature is from560° C. to 650° C.
 4. The method according to claim 1, where thesubstrate is LaAlO₃, in single crystal form.
 5. The method according toclaim 1, where the addition of H₂ O vapor provides a total H₂ O vaporpressure of from 7×10⁻³ mbar to 30×10⁻³ mbar.
 6. The method according toclaim 1, where the sputtering gas also contains Ar or other inert gas,the reactor is selected from an rf magnetron and dc magnetron type, andthe addition of H₂ O vapor provides a total H₂ O vapor pressure of from17×10⁻³ mbar to 266×10⁻³ mbar.